Field of the Invention
The invention relates to a printing-machine roller, especially a dampening-unit roller, and preferably a metering roller having an outer cylindrical surface with a coating provided thereon.
In mechanical construction engineering, a demand exists for surface-coated cylinders which must have a coating with a hydrophilic property. In printing technology, for example, a dampening unit of a printing machine is equipped with a plurality of rollers by means of which a dampening-medium film which is as thin and uniform as possible is applied to a printing form of a printing-form cylinder. A particularly thin and uniform distribution of the dampening medium ensures a good print quality. Moreover, it is important that the feeding of the dampening medium take place without any introduction of printing ink into the dampening unit. With arrangements in which the inking-unit rollers of the printing machine are connected to the dampening-unit rollers via at least one intermediate roller, it is particularly critical if printing ink should enter the dampening unit.
The better the hydrophilic property of the surface of the printing-unit roller (dampening-unit roller) is, the thinner and more uniform is the formation of the dampening-medium film thereon.
Top-grade printing, for example, art printing, requires mostly an extremely thin dampening-medium film, for example, 5 .mu.m thick, surrounding the entire outer cylindrical surface of the roller. To form such a thin dampening-medium film, it has heretofore been necessary to admix alcohol or alcohol substitutes to the dampening medium in order to reduce the surface tension. Isopropanol or ethanol, for example, are used as the alcohol. For reasons of environmental protection and/or due to the regulations for the prevention of accidents it is to be expected that, in the future, a reduction in the use of alcohol or alcohol substitutes may be suggested or even required by law. At the same time, there remains a demand for top-grade prints for which an extremely thin and uniform layer of dampening medium is a prerequisite.
Heretofore, in addition to special wetting agents (alcohols), specific surface materials which have been subjected to a selective treatment have been used in order to obtain the best possible hydrophilic properties for the printing-machine rollers. It is has become known heretofore to use ground and/or polished stainless steel as surface material for such dampening-unit rollers. Moreover, it has become known heretofore to galvanically deposit onto the surface of rollers or cylinders a chromium layer which is then ground and/or polished. In connection with the use of alcohols or alcohol substitutes admixed in the dampening medium, good print results have been obtained with the dampening-unit rollers described hereinbefore. If the amount of alcohol or the amount of the alcohol substitute is reduced, however, the print results are no longer satisfactory.
It is therefore an object of the invention to provide a printing-machine roller of the foregoing general type which has exceptionally good hydrophilic properties.